Microphysical and radiative changes in cirrus clouds by geoengineering the stratosphere

Abstract

[1] In the absence of tangible progress in reducing greenhouse gas emissions, the implementation of solar radiation management has been suggested as measure to stop global warming. Here we investigate the impacts on northern midlatitude cirrus from continuous SO 2 emissions of 2-10 Mt/a in the tropical stratosphere. Transport of geoengineering aerosols into the troposphere was calculated along trajectories based on ERA Interim reanalyses using ozone concentrations to quantify the degree of mixing of stratospheric and tropospheric air termed "troposphericity". Modeled size distributions of the geoengineered H 2 SO 4 -H 2 O droplets have been fed into a cirrus box model with spectral microphysics. The geoengineering is predicted to cause changes in ice number density by up to 50%, depending on troposphericity and cooling rate. We estimate the resulting cloud radiative effects from a radiation transfer model. Complex interplay between the few large stratospheric and many small tropospheric H 2 SO 4 -H 2 O droplets gives rise to partly counteracting radiative effects: local increases in cloud radiative forcing up to +2 W/m 2 for low troposphericities and slow cooling rates, and decreases up to -7.5 W/m 2 for high troposphericities and fast cooling rates. The resulting mean impact on the northern midlatitudes by changes in cirrus is predicted to be low, namely < 1% of the intended radiative forcing by the stratospheric aerosols. This suggests that stratospheric sulphate geoengineering is unlikely to have large microphysical effects on the mean cirrus radiative forcing. However, this study disregards feedbacks, such as temperature and humidity changes in the upper troposphere, which must be examined separately.Citation: Cirisan, A., P. Spichtinger, B. P. Luo, D. K. Weisenstein, H. Wernli, U. Lohmann, and T. Peter (2013), Microphysical and radiative changes in cirrus clouds by geoengineering the stratosphere,

Sulfate geoengineering: a review of the factors controlling the needed injection of sulfur dioxide

“…This means that the SG-driven increase of IN number density has basically no effect on the population of ice particles, but we may expect some impact on the ice particle size due to the larger size of IN made available by SG. This is the main conclusion of Cirisan et al (2013), who note that the more large geoengineered particles exist (of typical sizes close to 0.5 µm), the less particles have to struggle against the Kelvin effect and more droplets may grow to larger sizes. This study analyzes in detail the direct SG impact on IN, as a complementary effect with respect to the dynamical indirect effect investigated by Kuebbeler et al (2012).…”

“…This study analyzes in detail the direct SG impact on IN, as a complementary effect with respect to the dynamical indirect effect investigated by Kuebbeler et al (2012). The main conclusion of Cirisan et al (2013) is that the microphysical impact on cirrus clouds from geoengineered stratospheric sulfate aerosols is not an important side effect. They estimate a resulting midlatitude average RF in the range of +0.02 to −0.04 W m −2 , depending on upwelling velocities and geoengineering scenario.This is consistent with the conclusions by Karcher and Lohmann (2002), who found that the effect of a perturbed aerosol size distribution on the ice particle population formed via homogeneous freezing is of secondary importance.…”

“…Cirisan et al (2013) found that SG directly provides ice nuclei (IN) of a larger size with respect to those in the unperturbed atmosphere, resulting in a rather small increase in cirrus cloud coverage. Kuebbeler et al (2012), on the other hand, found that SG would decrease cirrus cloud coverage because of changes in temperature, vertical velocity and water vapor updraft.…”

“…They estimate a resulting midlatitude average RF in the range of +0.02 to −0.04 W m −2 , depending on upwelling velocities and geoengineering scenario.This is consistent with the conclusions by Karcher and Lohmann (2002), who found that the effect of a perturbed aerosol size distribution on the ice particle population formed via homogeneous freezing is of secondary importance. It should be considered, however, that the estimates from Cirisan et al (2013) are based on box model simulations and radiative transfer model calculations, and do not consider the dynamical impact and the feedback to microphysics.…”

“…In this case, when the ice supersaturation ratio exceeds approximately 1.1, heterogeneous freezing may start (Hendricks et al, 2011); sulfate aerosols do not act as potential IN in this case. Kuebbeler et al (2012) and, indirectly, Cirisan et al (2013) have demonstrated that only the indirect dynamical perturbation induced by SG may be capable of significantly perturbing the number density of upper-tropospheric ice particles, with decreased vertical velocities due to the enhanced atmospheric stabilization. As noted in Kuebbeler et al (2012), the idea proposed in some studies that volcanic eruptions may lead to enhanced ice crystal number concentrations was indeed confirmed by ISCCP lidar measurements (Sassen et al, 2008), whereas modeling studies found only a weak aerosol effect even in the case of large perturbations Lohmann and Feichter, 2005).…”